After versioning, functionality and UI they now copy sandbox-mode from the Chromium project. I don't think there's much time left before you won't be able to make the distinction between Firefox and Chromium anymore.
Is this really different than anything else in "pop programming culture?" Consider the spread of various paradigm practices in the different runtimes.
And this is not a condemnation of the practice. Just saying it really isn't that different than most anything else.
I will say that it is not that troubling. Far more troubling would be either a) multiple completely inconsistent browsers dominating the field or b) a homogenous ecosystem in the web. At least, I think so.
From what I've heard from both Google and MS people, Chrome had it first, it's just IE announced it (and shipped it in a beta) first. Alternatively, if you don't want to trust word of mouth — there's no way the Chrome team did their entire sandboxing implementation in the five months between IE announcing it and the announcement of Chrome, it's just too much work.
